There are many examples of drug delivery systems which operate using gas pressure to deliver a liquid such as a nutritive liquid at a controlled rate. U.S. Pat. No. 5,398,851 discloses a device comprising a casing having a liquid-tight membrane disposed therein which divides the interior of the casing into a propellant chamber and a liquid chamber or reservoir for the liquid to be delivered. The reservoir is in communication with an outlet. The propellant chamber contains two chemicals such as sodium bicarbonate and citric acid which are initially separated from one another by a barrier. Delivery is effected by breaking or perforating the barrier to allow the acid and bicarbonate to come into contact and thereby generate carbon dioxide gas. The carbon dioxide expands the propellant chamber by pushing on the membrane, thereby contracting the reservoir and causing liquid to be delivered from the casing via the outlet port.
Patent application Ser. No. 950366 discloses a feedback-controlled liquid delivery device which generates gas only as it is needed and therefore has advantages over the device of U.S. Pat. No. 5,398,851 due to the fact that it can maintain gas generation for a relatively long period of time (the device of U.S. Pat. No. 5,398,851 has a gas generator which, when actuated, proceeds until it is exhausted, even if delivery is stopped).
However, the devices of both U.S. Pat No. 5,398,851 and patent application Ser. No. 950366 each maintain a predetermined pressure within the system to drive the liquid from a reservoir, within the device, to a patient.
Neither the device disclosed in U.S. Pat. No. 5,398,851 nor the device disclosed in patent application Ser. No. 950366 can guarantee a predictable pumping rate under all conditions. This is because the viscosity of a liquid to be delivered must be considered when dealing with delivery devices containing an outlet of fixed size. If the liquid reservoir is pressurised to a predetermined pressure and the liquid is forced through an outlet of a fixed size, then it is the viscosity of the liquid which determines the rate of delivery through said outlet. The viscosity of a liquid can vary due to the influence of outside parameters such as temperature, and problems can arise during manufacture in producing a liquid of constant viscosity.
A further disadvantage associated with the device of U.S. Pat. No. 5,398,851 is that it requires a reservoir which can withstand the driving pressure of the generated gas and this eliminates the use of conventional bottles or bags which are not adapted to withstand elevated internal pressures.